The present invention relates to an epitaxial grown layer structure of III-V system compound semiconductor crystals and more particularly to an epitaxial grown layer structure of the crystals provided on a GaAs substrate doped with In.
It is anticipated that the GaAs substrate doped with In will be mainly used in the future as a substrate for GaAs IC's for the following reasons. First, the In doped GaAs is excellent in its crystallinity. The defect density of the GaAs doped with e.g., 0.1 to 0.5 atomic % of In (that can also be regarded as a mixed crystal In GaAs) is smaller by two or three orders of magnitude than the ordinary GaAs. Secondly, the threshold voltage Vth of MESFETs formed in the In doped GaAs substrate does not almost vary in a wafer and among wafers. Thirdly, the In doped GaAs substrate will contribute to implement heterojunction devices on elements at low cost. High speed elements of a heterostructure made through epitaxial growth have been proposed in U.S. Pat. No. 4,424,525, for example, and particularly elements of a heterostructure formed by GaAs and Al.sub.x Ga.sub.l-x As will be put into practical use in the near future. However, for wider practical use of such hetero-junction elements, a hurdle of implementing them at a lower cost must be jumped over. The In doped GaAs substrate will greatly contribute to obviate this hurdle since it can be industrially used in large quantities and is excellent in its quality.
The inventors of the present invention attempted an epitaxial growth of GaAs and Al.sub.x Ga.sub.l-x As on the In doped GaAs substrate through MBE (Molecular Beam Epitaxy) and MOCVD (Metal Organic Chemical Vapor Deposition) techniques. As a result, so-called cross-hatched patterns were produced due to the lattice-mismatched structure. The carrier mobility was only reduced by ten percent or so compared with the case of using an In undoped GaAs substrate. The emission intensity measured through photoluminescence phenomenon in the case of the GaAs substrate doped with 17 atomic % of In was 1/10 of the case of the In-undoped GaAs substrate, which means that the crystallinity of the epitaxial layers was apparently deteriorated.